This invention relates to solar energy systems in general and more particularly to an improved concentrating lens arrangement for use in solar energy systems.
Various solar energy systems have been developed. Of particular interest are the types of systems disclosed in U.S. Pat. No. 4,134,393 and in Ser. Nos. 866,067 filed Dec. 30, 1977, 866,068 filed Dec. 30, 1977, 806,291 filed June 15, 1977, 845,862 filed Oct. 31, 1977, 807,513 filed June 20, 1977, 915,001 filed June 13, 1978, 920,288 filed June 29, 1978, and 1,175 filed Jan. 5, 1979 all of which are assigned to the same assignee of the present invention.
A number of these systems utilize Fresnel lenses made of waterwhite glass in the largest sizes available throughout the world. For example, they are up to 86 cm in width by 240 cm in length. Fresnel lenses are particularly attractive for concentrating solar energy because of their low price. This permits systems of the nature described in the aforementioned patent and applications to produce energy more economically than other previously existing solar systems. In the aforementioned patent and applications there are various uses to which the solar energy which is collected may be put. For example, it may be used for heating, refrigeration, distillation of salty water, generation of electricity with high efficiency utilizing photovoltaic cells and production of electricity or other work through the use of thermal energy, i.e., by means of turbines or other engines.
In a typical type of system disclosed therein, a Fresnel lens is supported above a conduit system which includes at least an inner and an outer conduit. The focus of the Fresnel lens is directed onto or below the conduit system so as to concentrate solar energy thereon. The conduit system normally remains fixed, with the Fresnel lens, its frame and an appropriate counterweight supported by rotation about the axis of the inner tube in the conduit so as to follow the movement of the sun from east to west during the day. Typically, the whole system is tilted as a function of the elevation of its location. Tilting may be varied throughout the year to take into account the varying angle of the sun.
Although Fresnel lenses have been found the most useful and economical, it has been discovered that the Fresnel lenses conventionally available have a higher efficiency of concentration in their central section than near the edges. This is primarily because the refractive prisms in the Fresnel lens cannot be made with sharp ends, thereby causing some dispersion of the solar rays as the number of prisms per unit width is increased toward the edges of the lenses. The efficiency of solar transmission is, for example, about 90% in the center, i.e., from the center of the lens for a distance of about 16 cm, 85% for about the next 12 cm, 80% for approximately the next 10 cm and then 70-75% toward the edges of the aperture.
It thus becomes evident that there is a need for improving the transmission efficiency of a Fresnel lens used in concentrating solar energy.